1. Field of the invention
This invention relates generally to a process for forming planar coupler devices for optical fibers and the devices formed thereby, and, more particularly, to a process for forming a graded index optical waveguide structure and the device formed by such a process.
2. Description of the Prior Art
Optical communication systems, in which messages are transmitted by carrier waves of optical frequencies that are generated by sources such as lasers or light-emitting diodes, are of much current interest because of the advantages they offer over certain other conventional communication systems, such as a greatly increased number of channels of communication and the ability to use other materials besides expensive copper cables for transmitting messages. An optical waveguide device is used to conduct or guide waves of optical frequencies from one point to another. The operation of an optical waveguide is based on the fact that when a medium which is transparent to light is surrounded or otherwise bounded by another medium having a lower refractive index, light introduced along the inner medium's axis is highly reflected at the boundary with the surrounding medium, thus producing a light guiding effect.
As the development of optical circuits proceeded, it became necessary to have structures which could couple, divide, switch, and modulate the optical waves from one waveguide device to another. A Y-coupler is an example of such a device which is formed in a "Y" shape and couples signals together or divides them apart.
More recent developments in optical circuits have produced a graded index optical fiber, i.e., an optical fiber with a refractive index that is progressively lower from the center of the fiber to the periphery. These graded index optical fibers are highly desirable because they have an increased information bandwidth and information handling capability as compared to conventional stepped-index optical fibers. In using such graded index optical fibers, there is also the need to have structures which can couple, divide, switch, and modulate the optical waves from one such waveguide device to another.
One process for forming such graded index optical couplers makes use of an ion exchange process as described, for example, in a publication by G. H. Chartier et al, in Electronic Letters, Vol. 13, pp. 763-764, (1977). By the process of Chartier et al, an eutectic mixture comprising 80 percent lithium sulfate (Li.sub.2 SO.sub.4) and 20 percent potassium sulfate (K.sub.2 SO.sub.4) is heated in an oxygen atmosphere to 580.degree. C. A sodium glass slide is suspended over the melt for 30 minutes to allow thermal equilibration with the melt. The slide is then dipped into the melt for 20 minutes, and again suspended over the melt for 10 minutes to avoid thermal shock. Planar waveguides having a depth of 100 micrometers may be made by this process. The coupler structures are formed by appropriately masking the glass slide, for example, with a thick (1-2 micrometers) film of aluminum, prior to dipping the slide into the melt. By such an ion exchange process, lithium ions from the eutectic mixture are exchanged for the sodium ions in the soda glass substrate, thus modifying the refractive index of the ion-exchanged region of the substrate.
There are, however, certain disadvantages to the above-described ion exchange process and the optical waveguide devices formed thereby. One major disadvantage is that the coupling efficiency between a waveguide device formed by an ion exchange process and a graded index fiber is somewhat low, i.e., typically being only about 65%. In addition, there are certain difficulties encountered in carrying out the ion exchange process. For example, at the elevated temperature used for the ion exchange process, lithium sulfate decomposes to lithium oxide (Li.sub.2 O), which is corrosive to silicon dioxide. The lithium oxide reacts with the silicon dioxide to form Li.sub.2 SiO.sub.3, with a resultant thinning of the glass substrate. In addition, surface cracks often form on the glass substrate during the ion exchange process, and such cracks produce scattering in the waveguide device formed.
It is the alleviation of the problems associated with this prior art ion exchange process and, further, the formation of an improved coupling structure for use with graded index optical fibers which has a high coupling efficiency, to which the present invention is directed.